The Pleiades are also called the seven sisters. Seven of the stars in this cluster are visible in the night sky with the naked eye. The Pleiades are easy to spot. They are bright and very close to each other.

Star Systems

Our solar system has only one star. But many stars are in
star systems
of two or more stars. Two stars that orbit each other are called a binary star system. If more than two stars orbit each other, it is called a multiple star system.
Figure
below
shows two binary star systems orbiting each other. This creates an unusual quadruple star system. The distance separating the two pairs is about the same as the distance from our Sun to Pluto.

This is an artist’s concept of HD 98800. This is a quadruple star system made of two binary star systems.

Star Clusters

Star clusters
are small groups of stars. A star cluster is smaller than a galaxy. There are two main types, open clusters and globular clusters. Both types are held together by gravity.

Open Clusters

Open clusters
are groups of up to a few thousand stars. The Jewel Box, shown in
Figure
below
, is an open cluster. Open clusters tend to be blue in color. They often contain glowing gas and dust. The stars in an open cluster are young stars that all formed from the same nebula.

These hot blue stars are in an open cluster known as the Jewel Box. The red star is a young red supergiant.

Globular Clusters

Globular clusters
are groups of tens to hundreds of thousands of stars. Gravity holds these stars tightly together. Globular clusters have a definite, spherical shape. They contain mostly old, reddish stars. Near the center of a globular cluster, the stars are closer together.
Figure
below
shows a globular cluster. The heart of the globular cluster M13 has hundreds of thousands of stars. M13 is 145 light years in diameter. The cluster contains red and blue giant stars.

The globular cluster, M13, contains red and blue giant stars.

Vocabulary

globular cluster
: A very tight group of stars, up to hundreds of thousands.

open cluster
: A group of up to a few thousand stars.

star cluster
: A small group of stars.

star system
: A group of two or more stars that orbit each other.

Summary

Two or more stars may orbit each other in a star system.

A group of stars, smaller than a galaxy, is a star cluster.

The two main types of star clusters are open clusters and globular clusters.